I received my annual flu shot back in October, and I have to say, I felt pretty good about myself. Though doctors constantly tell us to get vaccinated, most Americans don’t bother — a survey by the Centers for Disease Control and Prevention (CDC) in November indicated that just 36% of all Americans age 6 months and older will get the flu vaccine. That’s too bad — a simple flu shot can save the average person days of unhappy illness (and lost time at work or school) if they get exposed to the flu this winter. And for the very young, the very old and the immunocompromised, a flu vaccination might make the difference between life and becoming one of the 30,000 some Americans who die from influenza-related causes each year.

There’s just one problem: influenza is unpredictable. Flu viruses are notoriously promiscuous, which isn’t a judgment of their morals, but their genes. Flu viruses mutate constantly, sometimes on their own as they replicate inside an infected cell, and sometimes through recombination — directly swapping genes with other flu viruses. We need a new vaccination every year because the flu strains we might be exposed to over the winter differ from season to season. The flu shot itself is actually a trivalent vaccine, meaning that it protects against the three separate flu strains that experts believe are most likely to be circulating around the Northern Hemisphere this winter. If all goes well, you’re unlikely to be exposed to a flu strain that’s not covered by the vaccine.

Unless, of course, something unexpected occurs. And that may indeed be happening this winter.

Since the middle of August, the CDC has received 12 reports of human infections with a new flu virus — a swine influenza A strain called H3N2. So far the cases come from five states — Indiana, Iowa, Maine, Pennsylvania and West Virginia — and all but one of the affected individuals were below the age of 18. While the virus itself seems to have originated in pigs and initially jumped directly from swine to human beings, half of the cases had no documented contact with pigs, which means there’s at least minimal person-to-person transmission. Because there’s already a common seasonal flu called H3N2, health officials are calling the new strain H3N2v — meaning v for “variant.”

Twelve cases may not seem like much, especially since only three required hospitalization and no one so far has died. But in an article for Scientific American earlier this month, ace flu reporter Helen Branswell explained why the possible emergence of a new flu strain worries health officials — even as they fear overly alarming the public:

The cases leave public health authorities in the U.S. and elsewhere wondering if a new swine-origin flu virus is circulating at low levels among humans — and what needs to be done if that is indeed happening. (Read “Flu Factories” in the January 2011 Scientific American (preview) to learn why health authorities fear the next pandemic virus may emerge as a result of industrial farming practices.)

Given the mercurial nature of flu viruses — which can easily mutate into lethal pathogens — ignoring the new virus is not an option, even though to date there have been no deaths and most of the infections have produced only mild symptoms. But the widespread perception that the 2009 swine flu pandemic was much ado about nothing means health authorities risk further damage to their already battered credibility if they sound an alarm and this virus turns out to be a dud. And they know it.

Even if H3N2v doesn’t mutate into something more dangerous, if the strain does establish itself in the human population — which it hasn’t yet — it could worsen the usual flu season because the vaccine likely won’t offer much protection. Young children could be particularly vulnerable because they’re unlikely to have had much exposure to similar H3N2 viruses in the past — or any flu strains, depending on how young they are. By the same token, older people may have more resistance to the new strain — CDC testing indicates the H3N2v is similar to a human H3N2 strain that circulated in the mid-1990s. (Humans actually gave the H3N2 virus to pigs back then — which shows just how interconnected and vulnerable we all are.) If you’re old enough to have enjoyed swing music ironically — think Swingers-era Vince Vaughn — you can probably fight off the new swine flu.

In fact, what unfolds with H3N2v could be similar to what happened in 1977, when H1N1 flu viruses suddenly re-emerged — possibly through a lab accident — after a 20-year absence. The new H1N1 infected mostly younger people who had never been exposed to a similar flu strain before, while older people proved more resistant.